This description generally relates to a light emitting diode.
Typically, Gallium nitride (GaN)-based group-III compound semiconductor light-emitting diodes (hereinafter referred to as LEDs) have a broad band gap and an excellent reliability over diodes using other semiconductors, enabling to develop LEDs covering a wide range of light emitting spectrums from ultraviolet to infrared rays.
Recent advancement in technologies for semiconductor LEDs made of group-III nitride compound allows the diodes to be employed for various fields of commercial purposes.
Particularly, the GaN-based group-III compound semiconductor LEDs are widely used for sign boards, displays, backlights and electric bulbs, and application thereof is being gradually on the increase. It is therefore very important to develop high-end LEDs.
FIG. 1 is a cross-sectional view of group-III nitride semiconductor LEDs. The diode is formed by sequentially depositing on a sapphire substrate a buffer layer 110, an N—GaN layer 112, an active layer 113 and a P—GaN layer 114. Furthermore, mesa-etching is performed from the P—GaN layer 114 to part of the N—GaN layer 112, and a transparent electrode 115 is formed on a P—GaN layer 114. An N-type metal layer 117 is formed on a the mesa-etched N—GaN layer 112 such that, if a voltage is applied via N and P electrodes, electrons and holes flow into the active layer 113 from the N—GaN layer 112 and the P—GaN layer 114 to generate electron-hole re-combination and light.
However, there is a problem in that heat generated in the course of diode operation cannot be smoothly emitted to degrade the characteristic of the diode because the LEDs are fabricated from a sapphire substrate having a low thermal conductivity.
There is another problem in that electrodes cannot be formed on and under the LEDs and instead are formed at the same direction, as shown in FIG. 1, suffering a partial removal of an active layer. As a result, a light emitting area is reduced to make it difficult to realize high-quality LEDs having a high luminance. The number of chips generated from the same wafer is inevitably reduced, thereby complicating the fabricating processes involving two-times of bonding during the assembly.
There is still another problem in that the yield rate suffers due to facet mismatch between hardness of a sapphire substrate and a relatively softer GaN-based layer when the sapphire substrate is used in the lapping, polishing, scribing and breaking processes for separating a wafer into unit chips after LED chip process is finished on the wafer.
FIG. 2 is a conceptual diagram illustrating a phenomenon in which light emitted from a nitride-based compound LED is totally reflected and confined within the device according to the prior art.
First, light traveling between two media each having a different refractive index experiences reflection and transmission at an interface, and if an incident angle is equal or larger than a prescribed angle, the transmission is not realized and a total internal reflection occurs, at which time, the angle is called a critical angle.
In other words, if the light emitted from an active layer 153 of the nitride compound LED of the prior art travels toward a transparent electrode 155 at an angle larger than the critical angle, it is totally reflected and confined within the device, and absorbed into the epitaxial layer of the device and the sapphire substrate 151, thereby decreasing the external quantum efficiency.